1. Field of the Invention
The present invention relates generally to fluid pressure sensors, and particularly to capacitive fluid pressure sensors.
2. Technical Background
Capacitive sensors are used widely in a variety of medical and industrial applications to measure linear parameters such as force and pressure. For example, capacitive pressure sensors are used in blood pressure products. They can also be used to measure air or other fluid pressures.
In the conventional approach, the capacitive pressure sensor includes a pressure sensor assembly coupled to an electronic circuit. The pressure sensor assembly includes a port that connects the sensor to the environment. The sensor itself is constructed using a variable capacitor. In this design, one plate of the variable capacitor is formed by attaching a metal plate to a diaphragm. A fixed plate is held at a distance using spacer elements, that are fixed or adjustable. There are several problems with the conventional approach.
Fabrication is difficult and expensive. The variable capacitor is not an off-the-shelf component. The use of a metal plate attached to a diaphragm introduces measurement errors if the planar surface of the attached plate is not parallel to the planar surface of the fixed plate. Further, the attached metal plate has a relatively large mass. This substantially increases the sensor's susceptibility to errors caused by vibration, acceleration, and the sensor's orientation to the earth's gravitational field.
The electronic circuit used in the conventional approach employs a three-inverter oscillator circuit that converts the capacitance of the capacitive transducer to a square wave. The frequency of the square wave is easily measured by a microprocessor, or by some other means. FIG. 1 is an electrical schematic of a conventional three-inverter oscillator circuit. The circuit includes three inverter gates G1, G2, and G3. Typically, each gate includes protection diodes. The biggest problem with the circuit depicted in FIG. 1 is the conduction of the input protection diodes of the threshold detector stage G1. In order to mitigate the effects of the diode conduction, the conventional design employs resistor R3. Depending on its value, R3 either reduces or eliminates the diode conduction. However, as R3 reduces diode errors, it amplifies errors introduced by other components in the oscillator. The direct effect is increased sensitivity to changes in capacitance of the internal circuit at the input of gate G1, thus affecting frequency stability. The group delay of the low-pass filter created by R3 and the input capacitance of G1 causes the sensitivity to most other errors in the sensor to be increased. The ideal situation is where no delays are added to the signal path. The introduction of R3 also changes the effective threshold voltage (Vth) of gate G1. The conventional design has other problems as well. The circuit board that is used to support the electronics and the means used to support the plates of the variable capacitor C1 include dielectric material that contributes to inter-plate capacitance between the nodes of the circuit, especially between plates of the capacitor C1. Because the dielectric constant of the circuit board supports varies with temperature, the conventional sensor is sensitive to changes in temperature.
What is needed is a compact, inexpensive capacitive sensor that is easily fabricated using commonly available components. A sensor is needed that is not susceptible to errors caused by vibration, acceleration, and sensor orientation to the earth's gravitational field. A sensor is needed that includes an improved oscillator circuit that reduces the effects of diode conduction, including drift over temperature, without the errors introduced by the conventional design.